Microbiology, 2021

908 22 • Respiratory System Infections
Figure 22.10
(a) This micrograph of Streptococcus pneumoniae grown from a blood culture shows the characteristic lancet-shaped
diplococcal morphology. (b) A colorized scanning electron micrograph of S. pneumoniae. (credit a: modification of work by Centers for
Disease Control and Prevention; credit b: modification of work by Janice Carr, Centers for Disease Control and Prevention)
Pneumococci can be presumptively identified by their distinctive gram-positive, lancet-shaped cell
morphology and diplococcal arrangement. In blood agar cultures, the organism demonstrates alpha hemolytic
colonies that are autolytic after 24 to 48 hours. In addition, S. pneumoniae is extremely sensitive to optochin
and colonies are rapidly destroyed by the addition of 10% solution of sodium deoxycholate. All clinical
pneumococcal isolates are serotyped using the quellung reaction with typing antisera produced by the CDC.
Positive quellung reactions are considered definitive identification of pneumococci.
Antibiotics remain the mainstay treatment for pneumococci. β-Lactams like penicillin are the first-line drugs,
but resistance to β-lactams is a growing problem. When β-lactam resistance is a concern, macrolides and
fluoroquinolones may be prescribed. However, S. pneumoniae resistance to macrolides and fluoroquinolones
is increasing as well, limiting the therapeutic options for some infections. There are currently two
pneumococcal vaccines available: pneumococcal conjugate vaccine (PCV13) and pneumococcal
polysaccharide vaccine (PPSV23). These are generally given to the most vulnerable populations of individuals:
children younger than 2 years and adults older than 65 years.
Haemophilus Pneumonia
Encapsulated strains of Haemophilus influenzae are known for causing meningitis, but nonencapsulated
strains are important causes of pneumonia. This small, gram-negative coccobacillus is found in the pharynx of
the majority of healthy children; however, Haemophilus pneumonia is primarily seen in the elderly. Like other
pathogens that cause pneumonia, H. influenzae is spread by droplets and aerosols produced by coughing. A
fastidious organism, H. influenzae will only grow on media with available factor X (hemin) and factor V (NAD),
like chocolate agar (Figure 22.11). Serotyping must be performed to confirm identity of H. influenzae isolates.
Infections of the alveoli by H. influenzae result in inflammation and accumulation of fluids. Increasing
resistance to β-lactams, macrolides, and tetracyclines presents challenges for the treatment of Haemophilus
pneumonia. Resistance to the fluoroquinolones is rare among isolates of H. influenzae but has been observed.
As discussed for AOM, a vaccine directed against nonencapsulated H. influenzae, if developed, would provide
protection against pneumonia caused by this pathogen.
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